This invention relates to a bandgap reference circuit that operates with low voltage.
Bandgap reference voltage generators are used in DRAMs, flash memories and analog devices and are required to provide stable voltages over a wide range of voltage supplies and temperatures. Increasing demand for use of lower supply voltages will soon push the supply voltage below 1.25 Volts, the standard for which bandgap reference circuits are now designed. A conventional bandgap reference circuit includes three sections: a core where an input voltage is developed and conditioned, a bandgap generator, and a current generator. This circuit must operate with a supply voltage that is at least a few hundred millivolts (mV) above the desired bandgap voltage (≈1.25 Volts).
FIG. 1 illustrates a conventional bandgap reference circuit 10 having a core region 11, a bandgap generator region 21 and a current generator region 31. The core region 11 includes two PMOS transistors, 12 and 13, connected at their sources to a voltage supply 14 and connected at their drains to negative and positive input terminals of a first operational amplifier 15 whose output terminal is connected to the gates of the first and second transistors, 12 and 13. First and second matched bipolar transistors, 16 and 17, have collectors and bases connected to ground. The emitters of the first and second bipolar transistors, 16 and 17, are connected to the drain of the first PMOS transistor 12 and through a first resistor 18 to the drain of the second PMOS transistor 13, respectively.
The bandgap voltage generator region 21 includes a third PMOS transistor 22, with source connected to the voltage supply 14 and gate connected to the output terminal of the op amp 15. The drain of the third PMOS transistor 22 is connected through a second resistor 23 to the emitter of a third bipolar transistor 24, whose collector and base are grounded.
The current generator region 31 includes a fourth PMOS transistor 32 with sources connected to the voltage supply 14 and gate connected to an output terminal of a second op amp 34. A negative input terminal of the second op amp 34 is connected to the drain of the third PMOS transistor. A positive input of the second op amp 34 and the drain of the fourth transistor 32 are connected through a third resistor 35 to ground. The fifth transistor 33 serves as a source for a current Iout. This device requires two operational amplifiers, at least five PMOS transistors, and a supply voltage that is at least about 400 mV above a target bandgap reference voltage.
If the supply voltage is decreased to 1.2 V and below, the standard bandgap voltage of 1.25 V can no longer be maintained. What is needed is a bandgap reference circuit that allows operation with supply voltages as low as about 1 V, or preferably lower, and that has no more than one or two stable operating points.
These needs are met by the invention, which provides a bandgap reference circuit that operates with a supply voltage of about 1V and that has one stable operating point, unless all currents in the system are substantially zero initially. The invention uses only one operational amplifier, four PMOS transistors and one additional current path to ground in one embodiment. The core includes a current generator embedded therein.